Nagaeli
You have two choices. You can use an AH (Fiat 600 derivative) block, or an A112A1 block. THe FIA recognize the Fiat 600D head with the AH block, and the A112 head with the A112A1 block. When I last inquired you could NOT interchange heads between blocks. If you use the Fiat head then you will also need the 2-piece intake manifold for the 36DCD7 carburetor. If this same carburetor is used on the A112 head, then there is some confusion as to which manifold is legal.
In either case the bore is 65mm and the stroke is 74mm, resulting in 982cc. As the class limit is 1000cc, you could legally overbore .5mm and reach a displacement of 996cc.